Method of tinning a steel shell bearing



Patented Feb. 21, 1950 METHOD OF TINNING A STEEL SHELL BEARING John H.Clawson, Schenectady, N. Y., assignor to General Electric Company, acorporation of New York No Drawing. Application August 7, 1946, SerialNo. 688,965

1 Claim.

This invention is concerned with a method of partially tinning metallicarticles. More particularly, the invention is concerned With a method ofpartially tinning a metallic article which comprises shielding theportion of the article desired to be kept free of the tin plating withan organosubstituted polysiloxane resin, and thereafter dipping theshielded article in the tinning bath.

Several methods have been employed to mask a portion of a metallicarticle intended to be tinplated so as to prevent that portion of thearticle from being plated when the article is later dipped in the tinplating bath. One method comprises coating the portion of the articledesired to be protected from the tinning bath with an alkali silicate,more specifically sodium silicate, prior to the plating operation.However, there are several disadvantages which accompany the use ofsodium silicate as the masking (or rejecting) medium for the metallicarticle. Since the temperature of the tinning bath may be from about 300to 500 C. or higher, depending, e. g., on the exact constitution of thebath, it often happens that when any portion of the sodium silicatebecomes immersed in the bath, small pieces of the sodium silicate willchip oif and fall into the tinning bath thereby contaminating it. Thisis especially true because of the brittleness and poor coeflicient ofexpansion of sodium silicate which, when applied as a coating on ametallic article having a greater coefficient of expansion, causes thesodium silicate to chip away when the metal is caused to expand by anyextreme temperature change as is usually the case when a cold metal isdipped in a hot tinning bath.

The use of sodium silicate is also disadvantageous because of the pooracid and moisture resistance of the sodium silicate. After masking themetallic article with the sodium silicate, a length of time may oftenelapse before the article is actually tinned. During this period thesodium silicate, because of its extreme hygroscopicity, tends to absorbmoisture unless extreme precautions are taken. When later the article isdipped in the tinning bath, sputtering caused by the moisture containedin the sodium silicate results in flaking away of pieces of the film,thereby exposing unprotected portions of the article which it is desiredto keep free from tinning.

It is often necessary, and usually desirable, to immerse the metallicarticle in an acid pickling solution prior to the tinning operation,especially if any length of time has elapsed after a previous picklingoperation. Such an acid treatment yields a clean surface on which thetin can be Ill-) plated. Sodium silicate, because of its poor resistanceto acids, especially to dilute acid solutions, would obviously be at adisadvantage if it were necessary to dip the entire sodiumsilicatecoated metallic article into the acid pickling solution prior tothe actual tinning operation.

Ceramic coatings have also been employed as masking materials forpreventing the deposition of tin plate on portions of an article desiredto be tinned by dipping in a tinning bath. However, the use of ceramiccoatings as shielding materials also presents many difficulties. Inorder to get a sufliciently protective coating, it is necessary to put athick coating on the article desired to be masked. If the dimensions ofthe article are to be kept within close tolerances, the use of thickmasking coatings is obviously a disadvantage. In addition, the ceramiccoatings are often quite porous, thereby permitting absorption of suchliquid materials as, for example, pickling solutions, fiuxing solutions,etc, usually employed in the preparation of the article for dipping inthe tinning bath. On occasion, because of the porosity of the ceramiccoating, minute portions of the tinning bath have found their way ontothose portions of the article desired to be kept free of the tinplating. Moreover, ceramic coatings as masking media are susceptible tothe same objection as regards their moisture pick-up and theirbrittleness as was pointed out above in connection with the same defectinherent in the properties of sodium silicate.

I have now discovered that the above-discussed defects can besatisfactorily obviated if an organo-substituted polysiloxane resin (forbrevity hereinafter referred to as the polysiloxane resin) is employedas the masking material for the metallic article. These polysiloxaneresins, more particularly, hydrocarbon-substituted polysiloxane resins,have outstanding heat-resistance and heat-stability at elevatedtemperatures. At temperatures encountered in the tinning baths (fromabout 300 to 500 C. or more), the polysiloxane resins are admirablysuited for the stated purpose, since they are able to withstand theelevated temperatures for the short periods of time within which anypart of the shielded portion of the metallic article may happen to beimmersed in the molten tinning bath. In addition, the polysiloxaneresins have excellent moistureand acidresistance which renders themunaffected by any moisture medium or by the pickling baths ordinarilyemployed for that purpose. Due to the flexibility which can be obtainedwith the polysiloxane resins, there is not as great a danger 3 that theresinous coating will chip from the coated article under the extremetemperature changes, thereby to contaminate the tinning bath.

The term tinning bath or other expressions using the term tinning isintended to include within its meaning molten metal baths wherein atleast a preponderant amount of the molten materials comprises tin.Examples of materials that may be used in such baths are tin or alloysof tin which melt at a temperature of the order of 300 to 500 C.Examples of alloys corresponding to the above description are (inaddition to pure tin) alloys comprising tin and lead, alloys comprisingtin and antimony, alloys comprising tin and cadmium, alloys comprisingtin and copper, alloys comprising tin and zinc, alloys comprising tinand aluminum, etc.

Various organo-substituted polysiloxane resins, e. g.,hydrocarbon-substituted polysiloxane resins, may be employed in thepractice of my invention. These include hydrocarbon-substitutedpolysiloxane resins containing an average of from about 0.5 to less than2 hydrocarbon groups per silicon atom. High temperature-resistantthermoplastic polysiloxane resins are usually obtained when thepolysiloxane resin contains an average of from about 1.5 or 1.6 to lessthan 2,

-e. g., 1.6 to 1.9, hydrocarbongroups (e. g., methyl,

ethyl, phenyl, or total methyl and phenyl radicals, etc.) per siliconatom. It will of course be understood by those skilled in the art that iit is possible, by the application of sufficient heat, to convertso-called thermoplastic polysiloxane resins to a state whereby theircharacteristics and properties resemble those of heat-converted orthermoset polysiloxane resins.

In addition to the aforementioned polysiloxane resins, I may also employthe usual heat-hardenable or potentially thermosettinghydrocarbonsubstituted polysiloxane resins, e. g., polysiloxane resinscontaining an average of from about 0.5 to 1.3 or 1.4 hydrocarbon groups(e. g., methyl, ethyl, phenyl, or total methyl and phenyl radicals,etc.) per silicon atom. I prefer to employ as the shielding or maskingmedium a hydrocarbon-substituted polysiloxane resin containing anaverage of from about 1.0 to 1.8 hydrocarbon groups per silicon atom.More specific directions for the preparation of the aforementionedhydrocarbon-substituted polysiloxane resins may be found in Rochow U. S.Patents 2,258,218 to 2,258,222, assigned to the same assignee as thepresent invention. It will also be apparent that thehydrocarbon-substituted polysiloxane resins employed in this inventionmay be further modified by the admixture with them of thermosettingresins such as phenolic, urea and melaminealdehyde' condensationproducts, unsaturated alkyd resins, etc.

Various methods well known in the art may be e employed to practice myinvention. The portion of the article desired to be protected from 1 thetinning plate may be dipped in, sprayed, coated or brushedwith theparticular polysiloxane resin (or a solution of the polysiloxane resin)employed as the masking medium. The shielded article is then preferablyheated at an elevated temperature of the order of 150 to 300 C. forapproximately 10 to 30 minutes. The temperatures and times of heatingare not critical.

The main object in these steps is to drive off any solvent which may bepresent and to advance the polysiloxane resin to a more advanced stateof cure. This heat-treatment willordinarily convert heat-convertiblepol-ysiloxane resins to the substantially infusible and insoluble state,thus increasing their heat-resistance at more elevated temperatures.

It is customary to dip the masked article, prior to the plating step, inan acid picklin solution. The article may then be dipped for a shorttime in fiuxing solutions usually employed immediately preceding thetin-plating step. The shielded article is thereafter dipped in thespecific type of molten tinning bath employed for the occasion inaccordance with methods now well known in the art.

Inorder that those skilled in the art may better understand how thepresent invention may be practiced, the following example is given byWay of illustration and not by way of limitation.

Example A steel shell bearing was coated on the outside by brushing on ahydrocarbon-substituted polysiloxane resin containing an average ofabout 1.7 total methyl and phenyl radicals per silicon atom (dissolvedin toluene to about '70 per cent solids). The coated bearing was thenheated in a 150 C. oven for about 20 minutes to advance the resin to thesubstantially infusible and insoluble state. The bearing was then placedin a muriatic acid pickling bath for about 1 minute and thereafter in aflux (a bath used to prepare the article for the tin plating operation)comprising zinc chloride dissolved in Water. The bearing was immersed ina tinning bath containing pure molten tin for a few seconds (temperatureof the bath was about 400 C.). After exposure to air for a short time,it was observed that the hearing was satisfactorily coated with the tinplate on th inside and that no tin at all adhered to the surface of thebearing which had previously been coated withthe .polysiloxane resin.Neither the pickling solution, nor the flux solution, nor the tinningbath had perceptibly affected the polysiloxane coating on the outside ofthe bearing. This result followed 'even though the thickness of thepolysiloxane coating was less than 0.002 inch thick.

The use of a methyl-substituted polysiloxane resin containing an averageof about 1.5 methyl groups per silicon atom in place of the methylandphenol-substituted polysiloxane resin employed in the above exampleafforded the same protection against plating by the tin.

It is to be understood that this invention is not intended to berestricted to the particular polysiloxane resin nor tinning bathemployed in the above example, nor to the specific article masked andtinned. Other polysiloxane resins of the type previously described,including methyl-, ethyl-, propyl-, phenyl-, benzyl-, tolyltotalmethyland phenyl-, etc; substituted polysiloxane resins may also beemployed. This invention is especially applicable in the masking and tinplating of bearings, boxes, sheets of metal, i. 'e., metal to which tinplating Will adhere, decorative tinning, etc. After the tin platingoperation, the polysiloxane resin may easily be removed from the platedobject by abrasion or by various chemical means. However, by virtue ofthe fact that the polysiloxane resin can be present in such a thin layer(and still act as an effective masking agent), the polysiloxane coatingmay be allowed to remain on the metallic article. i

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

The method of partially tinning a steel shell bearing which comprises(1) applying to the outside of the bearing a thin coating of a methyland phenyl-substituted polysiloxane containing an average of about 1.7total methyl and phenyl groups per silicon atom, (2) heating the coatedbearing at a temperature of about 150 C. to advance the resinous coatingto the infusible and insoluble state, (3) placing the bearing in an acidpickling bath and then in a flux bath comprising water and zincchloride, and (4) clipping the treated bearing in a molten tinning bathmaintained at a temperature of around 400 C. to obtain an articletinned. on the inside and completely free of tin on the outside.

JOHN H. CLAWSON.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS

